This invention relates to a method of erasing an optical storage media. More particularly, this invention relates to a method for controlling the thermal environment of a dye polymer optical storage media to achieve a bulk erasure.
A conventional dye polymer optical disk is formed from a substrate, a retention layer, and an expansion layer. The expansion and retention layers each contain dyes specifically chosen for absorbing light at given wavelengths. The expansion layer contains a dye which will absorb light of a prescribed first wavelength while substantially reflecting light at other wavelengths. Accordingly, exposure to light at the first wavelength results in a heating of the expansion layer due to the absorbed light energy. The retention layer typically contains a percentage of the same dye as the expansion layer, but at a smaller percentage. Accordingly, exposure to light at the first wavelength results in a heating of the retention layer, though at a lesser rate than the expansion layer. In addition, the retention layer includes a percentage of another dye which will absorb light at a second prescribed wavelength. Accordingly, exposure to light at the second wavelength results in a heating of the retention layer due to the absorbed light energy.
Alternatively, a reflective layer may be included between the expansion layer and retention layer. For such an embodiment, the laser is input to the media from opposite sides of the media onto the expansion layer and retention layer, respectively. The reflective layer prevents the light entering from the expansion layer side from passing to the retention layer. Similarly, the reflective layer prevents light entering from the retention layer side from passing to the expansion layer.
To record data onto the optical disk, a laser beam having a particular "record" wavelength (e.g., the first wavelength corresponding to the dye of the expansion layer) is focused at an the area to receive data. The focused laser heats the expansion layer at such area causing a localized expansion (i.e. a bump) away from the substrate into the retention layer.
The focused laser also heats the retention layer, though not as much, as described above. The radiation heating by the laser, along with the conduction heating resulting from the contact with the expansion layer (e.g., the non-reflective layer embodiments), causes the retention layer to increase in temperature. As the temperature of the retention layer increases above its glass transition temperature (Tg), the local area of the retention layer transforms to a rubbery state deforming to accommodate the bump of the expansion layer. Because the retention layer cools quickly relative to the expansion layer, upon removal of the laser beam the retention layer cools below its glass transition temperature before the expansion layer has cooled. As a result, when the retention layer returns to the glass-like state, the bump in the expansion layer is still present and thus is fixed in the retention layer. The area of the expansion layer previously exposed to the laser beam is maintained in the stressed bump state even after cooling due to the non-separable contact between the retention layer and expansion layer.
One technique for erasing the "bump" is localized "spot" erasure. A laser beam at an "erase" wavelength (e.g., second wavelength corresponding to the dye of the retention layer) is focused at the bump to be erased causing the retention layer in the local area to be heated. The retention layer is heated above its glass transition temperature transforming the area to a rubbery state. The stressed expansion layer area may then return to its relaxed flat state pulling the retention layer area with it. Upon removal of the laser beam the retention layer, now in a flat unrecorded configuration, cools and transforms back to its glass-like state.
The use of a laser beam having a "record" wavelength to accomplish data recording and a laser beam having an "erase" wavelength to accomplish a spot erasure limits the use of the storage media to systems having two lasers or systems with the capability to control the laser source to generate a laser beam at alternative wavelengths. Thus, it is typical to limit products to being read only devices. Such is the case for most commercial optical CD players. Accordingly, there is a need for an alternate erasure method.
To accomplish erasure of an optical disk according to the spot erasure technique requires serially processing each bump to return the local area to a flat state. Thus, tracking of the entire media is needed to accomplish a erasure of the disk. For an audio CD, the erase laser beam must pass over the recorded data tracks sequentially tracking the recorded data "bumps". If tracking is improper errors result leaving data points which are not erased.
Spot erasure also requires strict control of the laser power and exposure time to relax the retention layer without overheating the expansion layer. In addition, overheating of the retention layer itself is a problem in that the polymer forming the layer may degrade altering the glass transition temperature and coefficient of thermal expansion. Non-uniform thermal conditions within the expansion layer or retention layer are to be avoided to prevent non-uniform erasures and degradation of the optical media.
A method for accomplishing a bulk erasure with each bump being erased at the same time rather than serially is needed. Moreover, a bulk erasure method providing a temperature controlled environment is provide for uniformity and avoid degradation of the optical media.